In recent years, a portable device, such as a digital camera, carries a lithium ion battery as a secondary battery. Because lithium ion batteries are prone to overcharge or overdischarge, the lithium ion batteries are arranged with a battery pack including a protection circuit for protecting the batteries from overcharge or overdischarge.
FIGS. 4 and 5 illustrate the composition of respective battery packs according to the related art. In the battery pack 1 of FIG. 4, a series circuit in which a capacitor C1 and a resistor R1 are connected in series is connected in parallel with a lithium ion battery 2. A positive electrode of the lithium ion battery 2 is connected to an external terminal 3 of the battery pack 1, and a negative electrode of the lithium ion battery 2 is connected to an external terminal 4 of the battery pack 1 through a pair of n channel MOS (metal oxide semiconductor) transistors M1 and M2 for current cutoff.
The drains of the MOS transistors M1 and M2 are connected to each other, the source of the MOS transistor M1 is connected to the negative electrode of the lithium ion battery 2, and the source of the MOS transistor M2 is connected to the external terminal 4. Body diodes D1 and D2 of the MOS transistors M1 and M2 are connected between the drain and the source of each of the MOS transistors M1 and M2 respectively in an equivalent manner.
A protection IC (integrated circuit) 5 is arranged in the battery pack 1 to include an overcharge detection circuit, an overdischarge detection circuit, and an overcurrent detection circuit. A power-source voltage Vdd from the positive electrode of the lithium ion battery 2 is supplied to the protection IC 5 through a resistor R1, and a power-source voltage Vss from the negative electrode of the lithium ion battery 2 is supplied to the protection IC 5. Thus, the protection IC 5 operates.
When an overdischarge or an overcurrent of the secondary battery is detected by the overdischarge detection circuit or the overcurrent detection circuit, the protection IC 5 sets a DOUT output signal to a low level and turns off the MOS transistor M1. When an overcharge of the secondary battery is detected by the overcharge detection circuit, the protection IC 5 sets a GOUT output signal to a low level and turns off the MOS transistor M2.
In the battery pack 1 of FIG. 5, a thermistor R3 is arranged further. One end of the thermistor R3 is connected to a terminal 6 of the battery pack 1, and the other end of the thermistor R3 is connected to the external terminal 4. At the time of charging, a predetermined voltage from a charging device is supplied to the terminal 6 of the battery pack 1 through a potential-divider resistor. The voltage of the terminal 6 changes because a resistance of the thermistor R3 changes according to a temperature of the battery pack 1. The voltage of the terminal 6 is detected and the charging device is controlled to stop the charging when the temperature of the battery pack 1 exceeds a predetermined temperature.
Patent Document 1 listed below discloses a battery pack in which a first diode disposed in a first direction and connected in series to a temperature protection element (PTC element) and a second diode disposed in a second direction opposite to the first direction and connected in parallel with the first diode and the temperature protection element (PTC element) are connected to a secondary battery. This battery pack is arranged so that, at the time of normal discharging of the secondary battery, a current flows through the second diode in the second direction only and does not flow through the first diode in the first direction. Thus, even when a temperature of the battery pack becomes high at the time of normal discharging, no current flows through the first diode, and it is possible to inhibit operation of the temperature protection element (PTC element).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-152580